The invention relates to the field of digital information signal reception and more specifically, to the field of maximum likelihood detection of bit sequences in such signals.
The invention relates to a partial response maximum likelihood (PRML) bit detection apparatus for deriving a bit sequence from an input information signal, includes
input apparatus (1) for receiving the input information signal,
sampling apparatus for sampling, at a predetermined sampling frequency, the input information signal at sampling instants ti so as to obtain sample values of the input information signal at the sampling instants ti, said sampling frequency having a relationship with a bit frequency,
calculation apparatus for
(a) calculating at a sampling instant ti for each of a plurality of states sj at the sampling instant, an optimum path metric value PM(sj,ti) and for determining for each of the plurality of states a best predecessor state at the directly preceding sampling instant tixe2x88x921, a state at the sampling instant identifying a sequence of n subsequent bits,
(b) establishing the best path from the state at sampling instant ti having the lowest optimum path metric value, back in time towards the sampling instant tixe2x88x92N via best predecessor states, established earlier for earlier sampling instants, to establish an optimum state at said sampling instant tixe2x88x92N,
(c) outputting at least one bit of the n bits of the sequence of bits corresponding to said established optimum state at said sampling instant tixe2x88x92N,
(d) repeating the steps (a) to (c) for a subsequent sampling instant ti+1.
The PRML bit detection apparatus is based on a finite state machine with states corresponding to specific n-bit sequences.
Earlier filed EP patent application no. 98203146.0, having a filing date of 18.09.98 (PHN 17088), describes an apparatus for deriving amplitude values for such PRMTL bit detection apparatus. The amplitudes are derived from an input information signal, which amplitude values can be used as reference levels for the states of a finite state machine, which are needed for the computation of the likelihood functional in the partial response maximum likelihood (PRML) bit detection apparatus.
PRML detection requires reference amplitude-levels for each state in the corresponding finite-state-machine (FSM), from which the likelihood of different paths is computed, given the sampled signal waveform. The well known Viterbi-algorithm enables very efficient computation of the most likely path. Each state of an n-taps partial response (PR) corresponds with one of the possible n-bits environments as shown e.g. in FIGS. 1 and 2. In standard PRML detection, an equalizer setting is chosen so that a simple symmetrical partial response is realized in the nominal situation of zero tilt of the disc with respect to the laser beam, i.e. with simple integer-valued coefficients. The above citations are hereby incorporated in whole by reference. A single equalizer may not be optimal in terms of timing recovery. In such case a solution with two equalizers can be implemented, with one equalizer for the timing recovery, and a second one to equalize to the partial response levels. The second one may be made adaptive so that channel fluctuations may be followed, if a robust control mechanism can be set-up, e.g. one that measures the obliqueness of the channel, eg. from the eye-pattern, and transforms this into an adaptation of the tap-values of the equalizer. Non-linearities such as a systematic asymmetry between marks and non-marks (which can be runlenght dependent) are also a problem to be dealt with and are not accounted for in standard PRML using a linear partial response.
The invention aims at providing an improved PRMEL bit detection apparatus, which has a lower complexity.
In accordance with a first embodiment of the invention, the apparatus for deriving a bit sequence from an input information signal, includes
input apparatus (1) for receiving the input information signal,
sampling apparatus for sampling, at a predetermined sampling frequency, the input information signal at sampling instants ti so as to obtain sample values of the input information signal at sampling instants ti, the sampling frequency having a relationship with a bit frequency,
calculation apparatus for
(a) calculating at a sampling instant ti for each of a plurality of states sj at the sampling instant, an optimum path metric value PM(sj,ti) and for determining for each of the plurality of states a best predecessor state at the directly preceding sampling instant tixe2x88x921, a state at the sampling instant identifying a sequence of n subsequent bits,
(b) establishing the best path from the state at the the sampling instant ti having the lowest optimum path metric value, back in time towards the sampling instant tixe2x88x92N via best predecessor states, established earlier for earlier sampling instants, to establish an optimum state at sampling instant tixe2x88x92N,
(c) outputting at least one bit of the n bits of the sequence of bits corresponding to the established optimum state at the sampling instant tixe2x88x92N,
(d) repeating steps (a) to (c) for a subsequent sampling instant ti+1, characterized in that n is larger than 3,and that sequences of n subsequent bits having nxe2x88x921 directly successive bits of the same binary value are allocated to the same state. In a second embodiment of the invention, the apparatus for deriving a bit sequence from an input information signal, includes
input apparatus (1) for receiving the input information signal, sampling apparatus for sampling, at a predetermined sampling frequency, the input information signal at sampling instants ti so as to obtain sample values of the input information signal at sampling instants ti, the sampling frequency having a relationship with a bit frequency,
calculation apparatus for
(a) calculating at a sampling instant ti for each of a plurality of states sj at the sampling instant, an optimum path metric value PM(sj,ti) and for determining for each of the plurality of states a best predecessor state at the directly preceding sampling instant tixe2x88x921, a state at the sampling instant identifying a sequence of n subsequent bits,
(b) establishing the best path from the state at the said sampling instant ti having the lowest optimum path metric value, back in time towards the sampling instant tixe2x88x92N via best predecessor states, established earlier for earlier sampling instants, to establish an optimum state at sampling instant tixe2x88x92N,
(c) outputting at least one bit of said bits of the sequence of bits corresponding to established optimum state at sampling instant tixe2x88x92N,
repeating said steps (a) to (c) for a subsequent sampling instant ti+, characterized in that the calculation apparatus is adapted to obtain said optimum path metric value for the state at the sampling instant ti in step (a) by
(a1) comparing the optimum path metric values of all possible predecessor states at the directly preceding instant tixe2x88x921 of the state at the instant ti,
(a2) select the predecessor state at the directly preceding instant tixe2x88x921 having the smallest optimum path metric value as the best predecessor state,
(a3) combining the optimum path metric value of the best predecessor state at the directly preceding sampling instant tixe2x88x921 and a branch metric value corresponding to the state at the instant ti, so as to obtain the optimum path metric value for the state, the branch metric value for the state being obtained from the sample value at the sampling instant and a reference amplitude, which reference amplitude has a relationship with the state.
The invention is based on the following recognition. With the apparatus in accordance with the first embodiment of the invention, the number of states have been decreased significantly. This results in a reduced complexity in the calculation for finding the most likely path in the corresponding finite state machine. With the apparatus in accordance with the second embodiment of the invention claim 6, the complexity is also reduced, for the reason that the add-compare-select strategy normally carried out in PRML detection systems has been replaced by a simpler compare-select-add strategy.